1. Field of the Invention
The present invention relates generally to an optical disk used for information recording or reproduction, its substrate, and a mold for forming the substrate.
2. Related Background Art
Conventionally, an optical disk and its substrate have been proposed in a minidisk form disclosed in JP 5(1993)-303768 A.
FIG. 11 shows a schematic sectional view of a conventional optical disk. In FIG. 11, numeral 101 indicates an optical disk substrate, numeral 102 a handling surface, numeral 103 an information surface, numeral 104 a clamp plate retaining surface, numeral 106 a clamp plate, numeral 107 a groove, numeral 108 an information recording layer, numeral 109 a protective coating, and numeral 120 a rotation central axis. The functions of the optical disk configured as described above and its substrate are described as follows.
On the information surface 103 formed on one principal plane of the optically transparent disc-like substrate 101 made of polycarbonate, acrylic resin, or the like, a signal recording/reproduction layer or a reflection film is formed as the information recording layer 108 and the protective coating 109 is formed thereon. Thus, the optical disk is configured. The disc-like clamp plate 106 formed of an iron plate or magnetic stainless steel is attached to the clamp plate retaining surface 104 around the center of the substrate 101. The clamp plate 106 is attracted magnetically to a disk table fixed to the tip of a spindle axis of a recording/reproducing device. Thus, the substrate 101 is rotated together with the disk table.
For instance, JP 5(1993)-89529 A describes a welding method using ultrasonic waves and an adhesion method using an ultraviolet curable resin as methods of holding the clamp plate 106 to the substrate 101.
The welding method using ultrasonic waves is carried out as follows. As shown in FIG. 11, the clamp plate 106 is placed on the clamp plate retaining surface 104. Then, a ring-like contact face of an ultrasonic horn tip is pressed against an inner circumferential side end portion of the handling surface 102 with a predetermined force. In this state, ultrasonic vibration is applied, so that the resin forming the portion against which the contact face has been pressed (hereinafter referred to as the xe2x80x9cpressed portionxe2x80x9d) is melted. The pressing force of the ultrasonic horn allows the resin thus melted to deform and the pressed portion to be displaced downward. In addition, the melted resin projects from a wall surface extending between the clamp plate retaining surface 104 and the handling surface 102 and thus a projection part 110 as shown in FIG. 12 is formed. In FIG. 12, numeral 112 indicates a displaced surface formed by the displacement of part of the handling surface 102 on its inner circumferential side by the contact with the contact face of the ultrasonic horn. In this case, the projection amount of the projection part 110 toward the center corresponds to a displacement amount of the displaced surface 112 with respect to the handling surface 102. As a result, an outer circumferential side flat portion (hereinafter referred to as an xe2x80x9couter flat portionxe2x80x9d) 106a around the outer circumference of the clamp plate 106 is sandwiched between the clamp plate retaining surface 104 and the projection part 110. Therefore, the clamp plate 106 is retained so as not to come off the clamp plate retaining surface 104.
The adhesion method using an ultraviolet curable resin is carried out as follows. Initially, an ultraviolet curable resin is applied between the clamp plate retaining surface 104 and the outer flat portion 106a around the outer circumference of the clamp plate 106. Then, the ultraviolet curable resin is irradiated with ultraviolet rays to be cured. Thus, the clamp plate retaining surface 104 and the clamp plate 106 are bonded and fixed.
The substrate 101 is manufactured by injection molding. The handling surface 102 functions as a suction face to be subjected to the suction of a vacuum suction pad attached to the end of a robot arm when the substrate 101 molded by injection molding is taken out from the mold using the robot arm. The inner circumferential side end of the handling surface 102 is defined by the outer circumferential side end of the clamp plate retaining surface 104 and the outer circumferential side end of the handling surface 102 is defined by the inner circumferential side end of the groove 107. The groove 107 is formed as the reflection of the shape of a mold part required for holding a stamper incorporated into the mold. The inner circumferential side end of the information surface 103 (the inner circumferential side end in an area where grooves or pits are formed) is positioned at a predetermined distance from the outer circumferential side end of the groove 107.
In the optical disk and its substrate, it is required to reduce the diameter and thickness of the disk so that the optical disk can be applied to portable equipment.
However, in the optical disk and its substrate with the clamp plate 106 sandwiched between the clamp plate retaining surface 104 and the projection part 110 as shown in FIG. 12, there has been the following problem in thickness reduction. In order to reduce the thickness of the substrate 101, it is necessary to shorten the distance in the thickness direction between the handling surface 102 and the clamp plate retaining surface 104. In FIG. 12, therefore, the displacement amount in the thickness direction of the displaced surface 112 with respect to the handling surface 102 is reduced. Thus, the projection amount of the projection part 110 is reduced. As a result, the reliability in retaining the clamp plate 106 decreases.
In order to solve the above mentioned problem, the following method might be considered. That is, a protrusion 114 is formed along the inner circumferential side end of the handling surface 102 as shown in FIG. 13, and then while the contact face of an ultrasonic horn is pressed against the protrusion 114 to suppress the occurrence of the difference in level between the handling surface 102 and the displaced surface 112, the projection amount of the projection part 110 toward the center is increased. However, in this method, due to the formation of the protrusion 114, the inner circumferential side end of the handling surface 102 is displaced radially outward. In order to take out the substrate 101 from the mold used for injection molding, it is necessary to form the handling surface 102 with a larger area than a predetermined area. Therefore, the outer circumferential side end of the handling surface 102 is required to be shifted radially outward. Accordingly, the inner and outer circumferential side ends of the groove 107 and the inner circumferential side end of the information surface 103 also are required to be shifted sequentially to the radially outer side. Therefore, in order to secure the same recording capacity, the outer diameter of the optical disk increases, which has been a problem.
On the other hand, in an optical disk and its substrate in which the clamp plate 106 is bonded and fixed to the clamp plate retaining surface 104 with an ultraviolet curable resin, the above-mentioned problem is not caused when the size and thickness of the disk are intended to be reduced. However, after being applied, the ultraviolet curable resin is solidified gradually with the passage of time. For instance, therefore, in the case where the environmental temperature varies, since the clamp plate 106 and the substrate 101 are different in material and thus in thermal expansion coefficient, the substrate 101 is distorted, which may cause noise generation.
The present invention intended to solve the above-mentioned problem. It is an object of the present invention to provide an optical disk and its substrate whose sizes and thicknesses can be reduced and to provide a mold for forming the substrate. In addition, the present invention is intended to provide an optical disk in which no distortion remains in its substrate, and to provide the substrate for the optical disk.
In order to achieve the above-mentioned objects, the present invention employs the following configurations.
A first optical disk substrate of the present invention includes at least a clamp plate, a clamp plate retaining surface, a handling surface, and an information surface. The clamp plate retaining surface retains the clamp plate with an outer circumferential side flat portion of the clamp plate being in contact therewith. The handling surface is provided outside the outer circumference of the clamp plate retaining surface. The information surface is provided outside the outer circumference of the handling surface. At least part of the handling surface is inclined with respect to the information surface.
According to the first optical disk substrate, the top part of a slope of the handling surface is melted to be deformed, so that the clamp plate can be retained. As a result, it is possible to reduce the thickness of the optical disk and substrate with the clamp plate attached thereto. In addition, since the inner circumferential side end of the information surface can be positioned to be closer to the rotation central axis, an optical disk and a substrate with smaller outer diameters can be obtained.
In the first optical disk substrate, preferably, the substrate has a thickness in a range of 0.5 mm to 0.7 mm in an area where the information surface is formed. When the thickness is below the range, the mechanical strength of the substrate decreases and thus deformation of the substrate or surface wobbling during rotation may occur. On the other hand, the thickness exceeding the range results in the reduction in the working distance (a distance between an objective lens and the optical disk surface) with respect to an objective lens with a high numerical aperture of at least 0.6.
In the first optical disk substrate, it is preferable that an inclination angle of the handling surface with respect to the information surface is in a range of 2xc2x0 to 10xc2x0, further preferably 3xc2x0 to 6xc2x0. When the inclination angle is below the range, the inner circumferential side end of the information surface is positioned inevitably at a longer distance from the rotation central axis. Therefore, the recording capacity of the optical disk decreases or the disk diameter increases. The inclination angle exceeding the range causes the substrate to be partially thinner and thus the mechanical strength of the substrate decreases. In addition, malfunction of a suction pad in sticking to the handling surface may occur.
In the first optical disk substrate, it also is preferable that the handling surface has a radial width in a range of 1.8 mm to 2.5 mm. When the radial width of the handling surface is below the range, malfunction of the suction pad in sticking to the handling surface may occur. On the other hand, when the width exceeds the range, the inner circumferential side end of the information surface is positioned at a longer distance from the rotation central axis. Therefore, the recording capacity of the optical disk decreases or the disk diameter increases.
In the first optical disk substrate, preferably, the radial distance between an outer circumferential side end of the handling surface and the inner circumferential side end of the information surface is in a range of 2.0 to 4.0 mm. When the distance is below the range, the position to which a slider of a magnetic head can move on the inner circumferential side of the optical disk is limited and therefore the recording capacity is limited. On the other hand, when the distance exceeds the range, the inner circumferential side end of the information surface is positioned at a longer distance from the rotation central axis. Therefore, the recording capacity of the optical disk decreases or the disk diameter increases.
Furthermore, in the first optical disk substrate, it is preferable that a distance between a supposed first center plane and a supposed second center plane does not exceed 0.3 mm. The supposed first center plane is at equal distances from a surface of the clamp plate furthest from the clamp plate retaining surface and a back face of the optical disk substrate in an area where the clamp plate retaining surface is formed. The supposed second center plane is at equal distances from the information surface and a back face of the optical disk substrate in an area where the information surface is formed. When the distance between the supposed first and second center planes exceeds the range, the thicknesses of the optical disk and the substrate increase, which thus makes it difficult to reduce the thickness of a driving unit as compared to that of a conventional unit.
In the first optical disk substrate, preferably, a distance between the information surface and a plane including a circle defined by the deepest portion of the handling surface does not exceed 0.25 mm. When the distance exceeds 0.25 mm, the thickness of the substrate is reduced partially, and thus the mechanical strength of the substrate decreases.
In the first optical disk substrate, it also is preferable that an outer circumferential side end of the handling surface is lower than the information surface. This allows a stamper for molding the information surface to be held by its inner circumferential end.
Preferably, the first optical disk substrate further includes a projection part projecting toward the center formed inside an inner circumferential side end of the handling surface, with an upper face of the projection part being lower than the inner circumferential side end of the handling surface, and the clamp plate is limited in position in its thickness direction by the projection part and the clamp plate retaining surface. According to this configuration, the resin forming the inner circumferential side end of the handling surface is melted to form the projection part and thus the clamp plate can be retained. Therefore, an optical disk and a substrate can be provided in which distortion tends not to be caused by temperature variation.
In the first optical disk substrate, it is preferable that in the clamp plate, its outer circumferential side flat portion is thinner than its inner circumferential side flat portion. When the clamp plate is attached to the substrate with its thinner outer circumferential side flat portion being sandwiched between the clamp plate retaining surface and the projection part, the thicknesses of the optical disk and the substrate can be reduced. In addition, the inner circumferential side flat portion is formed to be relatively thick, so that a required magnetic attraction can be secured.
In this case, the inner circumferential side flat portion of the clamp plate may be formed of a plurality of stacked members. This allows the clamp plate having different thicknesses depending on the position to be manufactured easily at low cost.
A second optical disk substrate of the present invention includes at least a clamp plate, a clamp plate retaining surface, a handling surface, and an information surface. The clamp plate retaining surface retains the clamp plate with an outer circumferential side flat portion of the clamp plate being in contact therewith. The handling surface is provided outside the outer circumference of the clamp plate retaining surface. The information surface is provided outside the outer circumference of the handling surface. In the clamp plate, the outer circumferential side flat portion is thinner than its inner circumferential side flat portion.
According to such a second optical disk substrate, the clamp plate is attached to the substrate with the thinner outer circumferential side flat portion being held, so that the thicknesses of the optical disk and the substrate can be reduced. In addition, the inner circumferential side flat portion is formed to be relatively thick, so that a required magnetic attraction can be secured.
In the second optical disk substrate, the inner circumferential side flat portion of the clamp plate may be formed of a plurality of stacked members. This allows the clamp plate having different thicknesses depending on the position to be manufactured easily at low cost.
A third optical disk substrate of the present invention includes at least a clamp plate, a clamp plate retaining surface, a handling surface, and an information surface. The clamp plate retaining surface retains the clamp plate with an outer circumferential side flat portion of the clamp plate being in contact therewith. The handling surface is provided outside the outer circumference of the clamp plate retaining surface. The information surface is provided outside the outer circumference of the handling surface. The clamp plate retaining surface and the information surface are different in level. The thickness of the substrate in an area where the clamp plate retaining surface is formed is in a range of 0.75 to 1.25 times the thickness of the substrate in an area where the information surface is formed. The thickness of the substrate in an area where the handling surface is formed is in a range of 0.75 to 1.25 times the thickness of the substrate in the area where the information surface is formed. The distance between a wall surface extending between the clamp plate retaining surface and the handling surface and a wall surface extending between a back face of the clamp plate retaining surface and a back face of the information surface is in a range of 0.75 to 1.25 times the thickness of the substrate in the area where the information surface is formed.
According to such a third optical disk substrate, an optical disk and a substrate can be obtained in which mechanical and optical distortions hardly occur.
In the third optical disk substrate, it is preferable that at least part of the handling surface is inclined with respect to the information surface. According to this configuration, the top part of the slope of the handling surface is melted to be deformed, so that the clamp plate can be retained. As a result, the thicknesses of the optical disk and the substrate to which the clamp plate is attached can be reduced. In addition, the inner circumferential side end of the information surface can be positioned closer to the rotation central axis. Therefore, a small-diameter optical disk and substrate can be obtained.
An optical disk of the present invention includes at least the first, second, or third optical disk substrate and an information recording layer, and at least the information recording layer is formed on the information surface of the optical disk substrate.
Preferably, the optical disk further includes grooves along recording tracks or grooves forming recording tracks at the information surface, and a ratio of a quantity of light reflected from the grooves to a quantity of light reflected from a flat portion other than the portion where the grooves are formed is in a range of 0.3 to 0.8 when the information recording layer is irradiated with a beam. When the ratio exceeds the range, a sufficient rate of signal modulation in the grooves cannot be obtained. On the other hand, when the ratio is below the range, signals cannot be secured sufficiently due to a small quantity of light reflected from the grooves.
In the above, preferably, the beam with which the information recording layer is irradiated has the same wavelength as that of a beam used for recording and/or reproduction. This is intended to allow the condition described above to correspond to actual conditions.
A mold of the present invention includes at least a disc-like fixed-side main body with a flat principal plane, an inner stamper hold fitted inside the fixed-side main body, a fixed-side bushing fitted inside the inner stamper hold, a disc-like movable-side main body installed in a position substantially opposing the fixed-side main body at a predetermined space, a movable-side bushing, and a floating punch. The movable-side bushing is fitted inside the movable-side main body and is installed in a position substantially opposing the inner stamper hold at a predetermined space. The floating punch is fitted inside the movable-side bushing and is installed in a position substantially opposing the fixed-side bushing at a predetermined space. A surface of the inner stamper hold opposing the movable-side bushing is inclined with respect to the principal plane of the fixed-side main body. A surface of the fixed-side bushing opposing the floating punch is positioned on a side of the floating punch with respect to the principal plane of the fixed-side main body.
Since the surface of the inner stamper hold opposing the movable-side bushing is inclined, an optical disk substrate with a slope corresponding to the inclination can be formed. In addition, since the surface of the fixed-side bushing is positioned on the side of the floating punch, an optical disk substrate with a difference in level adapted to receive and retain a clamp plate can be formed. The top part of the slope of the optical disk substrate is melted to be deformed, so that the clamp plate can be retained. As a result, a thin and small-diameter optical disk and substrate can be obtained.
In the mold described above, it is preferable that the surface of the inner stamper hold opposing the movable-side bushing is a slope inclined from its inner circumferential side to its outer circumferential side toward the movable-side bushing. According to this configuration, an optical disk substrate can be formed which has a slope with its inner circumferential side higher. Therefore, the inner circumferential side end positioned at a higher level is melted to be deformed, so that the clamp plate can be retained.
Preferably, the above-mentioned mold further includes a disc-like stamper attached to the principal plane of the fixed-side main body, an outer circumferential side end of the surface of the inner stamper hold opposing the movable-side bushing being positioned on a side of the movable-side bushing with respect to a surface of the stamper, and an inner circumferential side end of the surface of the inner stamper hold opposing the movable-side bushing being positioned further apart from the movable-side bushing than the outer circumferential side end is. According to this configuration, the stamper can be held by the outer circumferential side end of the inner stamper hold.